Influence of grain size fluctuations on ductility of superplastic magnesium alloys processed by severe plastic deformation

Abstract

The influence of fluctuations in the grain size along the gauge length on ductility is analysed in the superplastic regime. It is demonstrated that these fluctuations produce a similar effect to that produced by variations in the initial uniformity of the sample, leading to premature necking. To reach superplastic elongations of 400%, fluctuations in grain size of <0·5% between two zones of the gauge length are required. As an example, the superplastic behaviour of an AZ61 alloy, processed by severe plastic deformation (SPD), with a heterogeneous microstructure is analysed when the grain boundary sliding mechanism controls deformation. It is found that neck formation is related to bands of fine grains that are formed during SPD processing by the mechanism of recrystallisation by rotation. Under these circumstances, grain refinement is rendered unsuccessful. The present investigation emphasises the importance of the microstructure homogeneity in developing grain refinement processing routes.